Behaviour change is one of those difficult subjects about which everyone has a view, but nobody has a simple solution. Getting people to change their ways is far from easy.

Government has long grappled with this, ever since the Post Office set up the first dedicated publicity unit in the 1850s to address the public's 'imperfect knowledge' of the, then government, department.

Successful campaigns - such as the drive to reduce smoking or efforts to improve road safety - show it takes sustained and strategic comms, using public information and advertising to get the message across.

Citizen journalism, 24-hour news, the massive growth in networking offered by what people call 'web 2.0' - all mean we live in a world where the media are more fragmented than ever before.

It is at times like this we need old style insights. To engage with people, we have to work out whom they trust.

There is a fascinating old piece of management research by David Krackhardt and Jeffrey Hanson. Writing in Harvard Business Review, they said the informal networks of trust in organisations bear little relation to the power or position that people hold.

When one CEO appointed a top performer to lead a strategic task force, it failed. Krackhardt recommended changing the top performer for someone who was bumbling along in the pack, but whom everyone trusted. The result? Success.

That is the challenge for public sector comms. We can't just target the people we want to engage. We have to work out whom they trust and get them to help us.

Emerging findings unveiled at a recent event run by the future trends think-tank Tomorrow Network suggests these 'trusted social networks' are key to either reinforcing or discounting public messages.

It is no longer audiences on which we need to focus. We have to find out where the trust lies.

Paul Mylrea is director of communications at the Department for International Development.